Use masks as part of materials in cycles
No, the MCE or UV/Image editor's masks will not translate upwards to the 3D view. 

They occur in the post phase of Blenders sequence of operations. Also I am told that its to hard to make a rasteriser for arbitrary material parameters. I guess they would have to operate at screen space?
You can see a more thorough breakdown of this at my blog.

The work around is to use Curve objects edited to an appropriate shape. You can use the tracker to map Hook objects in the camera view, these can be used to distort the curve object. Then project your texture onto the object as required.



Use an empty to control all dimensions.


You might notice that you cannot have a feathered edge for the same non rasterising reason. You can place a beveled glass object above then use its transmission ray to to fake a falloff, but you will be stuck with the shape of that object not your mask.